It is conventional practice to build a fire detection system into a building. Such systems usually include a control panel and detectors which are distributed throughout the building. Each detector is linked to the panel. The detectors are of various types such as smoke detectors, heat detectors, ionization detectors etc.
Smoke detectors take a number of forms. For example, an extensively used form of smoke detector comprises a light source and a light sensitive cell. The light source and the cell are misaligned in the sense that light from the light source cannot fall directly on the cell. For light to reach the cell, it must be reflected off particles in the air. As the quantity of smoke in the air increases, more light is reflected onto the cell with the result that the output of the cell increases. The panel is set to establish an alarm condition when the output of the cell reaches a predetermined value. For example, the panel can be set to establish an alarm condition when the quantity of smoke in the air reaches 3% per meter obscuration.
Fire detection systems are also known which include so-called aspirating high sensitivity smoke detectors. The smoke detector itself is mounted within a closed box and one or more pipes run from the box through the areas being protected. Each pipe has a plurality of holes in it. A fan evacuates the box so that air is drawn into the box through the pipes. The smoke detector is set so as to be far more sensitive than smoke detectors which are distributed throughout the building. Typically, high sensitivity smoke detectors are set to give an alarm condition when the smoke percentage reaches 0.1% per meter obscuration.
It will be appreciated that air is entering the pipes through a plurality of holes and that the pipe may run through a number of separate rooms. Should there be a fire in one of the rooms, then smoke will be drawn into the pipe through the holes which are in that room. However, uncontaminated air will be drawn into the pipe through holes which are in the other rooms. There is thus, in the pipe, a diluting effect. More specifically, the smoke which enters the pipe from the room in which there is a fire is mixed with clean air coming in from the other rooms. Thus, while the high sensitivity detector may be set to establish an alarm condition at 0.1% per meter obscuration in the detector box, there must be far more smoke in the burning room than that before the smoke percentage at the high sensitivity detector itself reaches 0.1% per meter obscuration.
Generally fire detection systems have to be made insensitive to low emission levels (whether it be smoke, ionized particles or heat) to prevent so-called nuisance alarms. The more sensitive the system is to low emission levels the more prone it is to establishing an alarm condition when there is no fire.